The present invention relates to a drying apparatus for continuous product webs which during drying contain a solvent becoming volatile, in particular for printed and/or laminated paper webs.
During drying of the above-mentioned product, the drying air must be maintained at a temperature of substantially 200.degree. C. in order to evaporate the solvent. The concentration of the solvent vapor in the dryer atmosphere must not exceed a predetermined value. Therefore waste gas must be aspirated from the dryer and replaced with a corresponding quantity of fresh air. The solvent vapors contained in the waste gas are converted by thermal combustion into non-damaging substances, in particular carbon dioxide and water. The considerable heat quantity which is released during the thermal combustion is partially recovered in known drying apparatuses. In particular pure gas heated over 700.degree. C. is partially recirculated into the drying and/or a heat carrier utilized for heating, which for example serves for heating the dryer atmosphere. The heat balance is improved with a reduction of the pure gas quantities discharged into the atmosphere and a reduction of their temperature.
One of the known drying apparatuses is disclosed for example in the German document DE-GM 81 15 801. In this apparatus the aspiration tube extending from the cooling tunnel opens into a channel portion which guides from the dryer to the secondary stage of the first heat exchanger. Due to the admixing of the air aspirated through the cooling tunnel, the waste gas is increased and cooled. A blower transports the waste gas mixture to the first heat exchanger and to the combustion chamber. The total pure gas stream discharged from the combustion chamber is first supplied through the primary side of the first heat exchanger and after cooling to substantially 500.degree. C. to the primary side of the second heat exchanger. From there the total pure gas stream after further cooling is expelled into the atmosphere. For obtaining high heat recovery with large gas quantities, large heat exchangers are required.
Another drying apparatus is disclosed in the German document DE-PS 26 16 347. It differs from the above-described drying apparatus in that has only one heat exchanger. A suction channel which extends from the cooling tunnel opens, as in the first cited patent document, into a waste gas channel which connects the dryer with the secondary side of the heat exchanger. From there the waste gas channel extends to an inlet of the internal combustion chamber and from its outlet through the primary side of the heat exchanger to a fireplace. A branching conduit leads back from the outlet of the combustion chamber to the nozzle box of the dryer. Therefore, the mass stream which flows at the secondary side through the heat exchanger is increased and cooled. The partial stream which flows through the primary side of the heat exchanger is cooled there for example to 300.degree. C. The primary side outlet temperature which is important for the heat losses cannot be lower than the secondary side inlet temperature. For obtaining a lower pure gas temperature, a larger heat exchange is required. A disadvantage of this known drying apparatus is that the gas stream which is supplied to the nozzle box is very hot, namely over 700.degree. C. The supply conduit and the nozzle cannot withstand such conditions for a long time.
The European patent document EP-A1-0 326 228 discloses an arrangement for heating a dryer and for burning the solvent vapors contained in the waste gas, in which a heat exchanger with a combustion chamber is integrated to an elongated, compact structural unit. The combustion chamber is connected with the heat exchanger in a joint housing and has two hair-needle shaped gas trains. The combustion chamber is provided with an outlet opening for a partial stream of the pure gas, which serves for heating the dryer temperature. The structural unit can be arranged inside the dryer. In this case, however, the dimensioning of the heat exchanger within certain limits is needed. Under the conditions of the practice the raised heat utilization must be lower than in the pre-heated drying apparatus.